Alright, here are some critics and ideas.
* The Delmai: Their myth has two problems:
First, it doesn't really differentiate much from Lucifer's myth, apart from being all the angels rebelling instead of the "best" one. They could use some unique source and objective, for example say they were men, like everyone else, but in an early age they got corrupted by something, and now seek to spread corruption among men, dragging them down with them (perhaps using the image of a drowning man pulling things down with him).
Second, nothing explains why /all/ of them became corrupted, nor why Elrin made them a place before they rebelled, much less why he made Kragmar to begin with. If you follow the common-source idea, the repentant Delmai could have become the peasantry, while those that stayed always faithful would be the nobles. Regarding Kragmar, it could be simply a place where men lived that got corrupted when they turned into Delmai, becoming their home. For such, Elrin separated it from the rest of the earth, and cast it apart. Just an idea.
* The Writ: Unfortunately, it strikes huge semblance to the Bible or the Quran. You could use the Celtic-Druid or Greek idea of a oral tradition, in the first example being its knowledge secret to everyone but the clergy, and taking from both there could be secret formulas and passages, and public ones. You can further mix it making the public passages written, and the secret ones forbidden to be written, for example. If you want to add the random factor, have one kind of divination process, perhaps with a combination of dice-like artifacts or with the observation of a natural factor. There are many options, really.
* Elrin: Give him a personality. Even the the Jewish god has a personality, and while the Christian and Muslim ones are more ethereal, they make it up with Jesus and Mohamed, who also had strong personalities. A way to do it is going primitive and creating a sort of genealogy.
First associate the god with a primitive phenomena/domain, like fire, shelter/protection, fertility, thunder, rain, (an) animal(s), the sea, pestilence, natural order, a patriarch, death, etc. Then go start giving that god more power, evolving the myth, until it becomes the sole god. In the end you will have a god that does cover all, but that has a distinct personality and perhaps even representation (a paternalistic and furious gods that punishes humanity for its excesses, receiving sacrifices; a protective god that intercedes for everyone one that prays for him, being worshipped specially during births; an absent god that only visits its children in the hour of death, and thus judgement; an evil god that is worshipped in order to redirect natural disasters to the enemies, and be spared; a powerful god that demands their children to live up to his image, proving themselves in combat/arts).
* The Taint: It is very similar to the Original Sin. It could be, alternatively, a corruption inherent to the universe, instead of a rebellion against Elrin. Being so, humanity/the Delmai move from "guilty cursed" to "self-victimized corrupter", for example. The taint is a hole they fell in, and repentance is the only way to save themselves from it. Unfortunately it is a vicious cycle, and past a certain point corruption can be considered complete. But... there is always a but, and myths and legends come out of them...
If you like the ideas, feel free to use them, and to discard them if you don't.
